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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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16th Feb 2018, 1:27 pm | #1 |
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Mullard 3 valve 3 watt amplifier circuits
Circuits - plural!
The 3-3 amplifier is well-known, gives amazing results (with a decent output transformer), and is loved by many. It uses a few novel circuit techniques, which makes it rather interesting. It wears dreamed up to showcase what could be achieved with the Mullard B9A audio valves EF86, EL84, plus the EZ80 rectifier. But there is another, slightly earlier, 3 valve 3 watt Mullard circuit, which uses two ECL80's in push-pull, plus EZ40 (could be EZ80). This is even simpler! Although it has no negative feedback. Question is, which of these two circuits has the greater potential as the basis for a good 3-watt amplifier? Has anyone built both and done any comparisons? The symmetry of the ECL80 amplifier, plus the valves all being in the same size bottle, does rather appeal visually! |
16th Feb 2018, 1:33 pm | #2 |
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Re: Mullard 3 valve 3 watt amplifier circuits
There is the compromise with the ECL80 of the common cathode, there is a Ferguson player amp circuit around.
Class A single end EL84 is simpler all round. Heaven forbid using EZ40, its a retrograde. |
16th Feb 2018, 4:32 pm | #3 |
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Re: Mullard 3 valve 3 watt amplifier circuits
It probably depends on what you want from the amplifier, good quality or HiFi quality.
Leaving the other specifications aside and just choosing the THD figures for comparison the EL84 single ended circuit wins out. |
16th Feb 2018, 6:13 pm | #4 |
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Re: Mullard 3 valve 3 watt amplifier circuits
I'd go for the single-ended EL84 version, if only because it's got a lower component-count!
Only 3 Watts from a single-ended EL84 though? I'ts significantly underachieving! With the right voltages you can easily get 5W from an EL84. And the EL84 is reasily availablle - £7.27+VAT from RS: https://uk.rs-online.com/web/p/doubl...alves/6784120/ |
16th Feb 2018, 7:07 pm | #5 |
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Re: Mullard 3 valve 3 watt amplifier circuits
There has been discussion on the push pull ECL80 amp before. The common cathode for triode and pentode does make for some unconventional circuitry, but it can be done. Apparently instability can be a problem.
Alan. |
16th Feb 2018, 9:50 pm | #6 |
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Re: Mullard 3 valve 3 watt amplifier circuits
... And Biggles points this out too. It gives (positive) feedback from pentode to triode by virtue of the common impedance to 0V of the two sections.
However, going Class A push-pull, and linking BOTH common cathodes together, the signal component of the cathode currents cancel out, giving almost no feedback. This is what Mullard did, to tame it all. The component count of the twin ECL80 amplifier is actually less than the 3-3! Though distortion is worse. But then there's no NFB. If I was to start playing with this, I'd include a tertiary winding in the transformer, close-coupled to the secondary, and connect (floating winding) this IN SERIES with the input, to introduce NFB, as there really isn't a suitable injection point. Last edited by kalee20; 16th Feb 2018 at 9:55 pm. Reason: Add last paragraph. |
17th Feb 2018, 5:58 am | #7 |
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Re: Mullard 3 valve 3 watt amplifier circuits
If you are going to the trouble of modifying or building a special output transformer, perhaps consider a single-ended ultralinear (SEUL) version of the Mullard 3-3. I think that the EL84 was used in SEUL mode by one or two makers, including Baird. Or further out in left-field, a “junior” version of the Pye Mozart circuit, with EL84 in place of EL34, and without the positive feedback (i.e. as in the HFS20, not the original HF10). And if stereo is required, an orthogonal version of a Mullard 3-3 could be considered; at least it would be push-pull in the sum (L+R) channel.
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17th Feb 2018, 7:41 am | #8 |
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Re: Mullard 3 valve 3 watt amplifier circuits
If I were building an amplifier and winding my own OPT and wanted to get the best possible low THD amplifier I wouldn't choose an ECL80 or a 3-3.
By using an ECL80 your making your life hard for no reason. The ECL valves were used to make cheap record player amps and are always a compromise. Class A SE isn't the way to go either IMHO, I could go into a lot of detail but suffice to say no reputable amp maker used SE, they all used PP for a reason. Using a valve rectifier in a hifi amp is a pointless. They drop voltage at high power, you want your supply to have good regulation and be stiff as. I've been reading a hell of a lot and building different amps these last two weeks, this article sums up my conclusions very well - http://www.angelfire.com/electronic/...-DB_Heart.html I copy the authors list here : - 1)Balanced drive to the output tubes is very important in order to maintain low distortion. 2) There should be only one RC low-frequency roll-off inside the global feedback loop. Failing that there should be as few as possible. 3)The fewer stages inside the feedback loop the better. 4)Stages ahead of the output tubes and inside the feedback loop should have as much head room as possible. I'd add another, stages ahead of the OP stage should have as low as poss open loop THD. I'm not dissing the 3-3 as such but it is an old design, it can be improved on, by, for a start triode strapping the EF86 or using an ECC83 instead. If your building an amp with what bits you have lying around, fair enough, but if you want very low distortion and are going to the trouble of winding your own OPT, using ECL80's and the 3-3 isn't the way. Andy.
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17th Feb 2018, 4:29 pm | #9 |
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Re: Mullard 3 valve 3 watt amplifier circuits
I am seriously considering building a couple of these for my 'home cinema' driving a couple of ls3a/5a 15 ohm speakers. A friend of mine built one of these amplifiers and it really is surprisingly good. What decent output transformers are available for these now? I believe the original used a special Parmeko transformer. Does anyone know the original Parmeko part number for them and can you still get these from Parmeko who I think still exist. Are the VVT ones any good in terms of the amount of 'iron' in them?
Thanks for your thoughts/comments in advance. Tim
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17th Feb 2018, 5:25 pm | #10 | |
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Re: Mullard 3 valve 3 watt amplifier circuits
Quote:
Cheers, GJ
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17th Feb 2018, 5:49 pm | #11 | |
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Re: Mullard 3 valve 3 watt amplifier circuits
Quote:
I certainly agree that balanced drive to the output bottles is needed if you're pursuing low distortion. With simple valve circuits and an output transformer, you aren't going to be able to use a lot of feedback, so you have to get your open loop distortion down. He makes a very good point that the design of loop poles and zeroes from purely the point of stability allows you to do things which are very bad from the point of recovery from transient overloads. It's important with valves and it's even more important with transistors. His statements about only having one time-constant or as few as possible is a bit too sweeping and misses out on the fact that complex systems with multiple roots can be designed, and designed to behave well even with stressful signals. He's missed the failing of the simple long tailed pair in that the path to the opposite anode is effectively cascoded, whereas the path to the same side anode is not cascoded and suffers worse from Miller effect. Also the concertina phase splitter is a cathode follower to one output and a common-cathode amp to the other, so the output impedances presented to the following stage are rather asymmetric. Oh and the bit about the quality of vacuum in low Earth orbit versus the quality of vacuum in a good valve once the getter has done its job is wrong. A long time ago I checked the real values. I made a quip that audiophiles ought to have new valves sent up, unsealed, to Mir, and left outside to outgas. A cosmonaut could go out and seal them off, to be returned with better-sounding natural vacuum. It was for April the first, many years ago. Only one person spotted the hidden joke in the residual pressures.... As far as single-ended amplifiers go, back in the day no respectable designer used them in any amplifier with any aspirations towards high quality, and those designers were not stupid. Single ended amps are a fashion item nowadays and fashion is immune to having its justification questioned. David
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17th Feb 2018, 6:42 pm | #12 | |
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Re: Mullard 3 valve 3 watt amplifier circuits
Quote:
I've never claimed to have 'golden ears' - the human ear isn't linear! hence I'm perplexed at those who seek sub-0.1%-THD amplifiers without appreciating that even the ideal speaker-to-free-air-to-human-ear interface is likely to introduce vastly greater nonlinearities than the amplifier once you turn the volume up. |
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17th Feb 2018, 6:53 pm | #13 | |
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Re: Mullard 3 valve 3 watt amplifier circuits
Quote:
Cheers, GJ
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17th Feb 2018, 7:18 pm | #14 | |
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Re: Mullard 3 valve 3 watt amplifier circuits
Quote:
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17th Feb 2018, 7:29 pm | #15 |
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Re: Mullard 3 valve 3 watt amplifier circuits
For audio-output transformers I can recommend Hammond:
http://www.hammondmfg.com/125SE.htm http://www.hammondmfg.com/125a.htm though for push-pull I've also used generic "20-Watt" 100V-line-output transformers for a 6BW6 amp with impressively-loud results. |
17th Feb 2018, 11:09 pm | #16 | |
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Re: Mullard 3 valve 3 watt amplifier circuits
Quote:
Colin |
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17th Feb 2018, 11:25 pm | #17 |
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Re: Mullard 3 valve 3 watt amplifier circuits
I did wonder about using the output stages (and output transformers) from Pye Fenman (Fenmen?)) 2 for this project. But I've heard a Mullard 3-3 and it appears to punch well above its weight. Hence my enquiry.
Tim
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18th Feb 2018, 8:12 am | #18 |
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Re: Mullard 3 valve 3 watt amplifier circuits
Yes the earth orbit stuff was a bit daft David but most of his other points are valid for a non proffesional designer.
"This may seem heresy to some but I don't have a problem with single-ended amplifiers" is depends what your trying to do, if you want to knock up a quick easy amplifier and not spend 1000's on valves and OPT's then they do the job. I knocked up a SE amp like the 3-3 and it sounded pretty good, though I found the OPT was pretty critical. I had to go through quite a few till it worked. Rod Elliott has some interesting things to say about valve amps and SE/SET amps. See - http://sound.whsites.net/valves/design.html Part 2 - Topology. Got you Colin, it's just that you mentioned winding your own OPT and also thought about using ECL80's which seem diametrically opposed. If anyone was building a 3-3 and wanted to experiment there's a few things you can play with using modern components like LED's as cathode resistors, active loads instead of anode resistors, or replace the EF86 with a LTC6090 - http://cds.linear.com/docs/en/datasheet/6090fe.pdf . that's got me thinking...... How about one of these OPT's Tim - http://www.emporiumhifi.com/new-hifi...-transformers/ : ) Happy building, Andy
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18th Feb 2018, 10:05 am | #19 |
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Re: Mullard 3 valve 3 watt amplifier circuits
A single-ended output stage, compared with push-pull, does have the interesting feature of not cancelling out even-harmonic distortion, which has a reputation of being sweet-sounding. Maybe this was part of the success of the Mullard 3-3.
Martin
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18th Feb 2018, 8:50 pm | #20 |
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Re: Mullard 3 valve 3 watt amplifier circuits
I don't want to go off topic obviously, but ISTR that one of the folk on this forum mentioned that the EL84 is a B9A version of a similar valve on an octal base. Sadly I can't remember what the valve was. I wondered if it would be possible to build a 3-3 using an EF37A and that valve and something like a 5U4 rectifier?
Tim
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